1. Field of the Invention
This invention relates generally to gas diffusion electrodes and, more particularly, this invention relates to gas diffusion electrodes adapted for use in electrochemical cells utilizing a liquid electrolyte and consuming or generating a gas via the electrochemical process occurring within the gas diffusion electrode.
2. Description of Related Art
The use of gas diffusion electrodes in fuel cells and metal-air batteries is well known. Gas diffusion electrodes have also been used in the electrolysis, either oxidation or reduction, of gaseous reactants. It is also possible to generate gases in such electrodes. In general, gas diffusion electrodes take the form of solid porous (gas and liquid permeable) bodies formed at least in part of an electronically conductive, electrochemically active material, and may include a catalyst. Such electrodes generally define an electrolyte contacting surface and a gas contacting surface. Electrochemical oxidation and reduction occurs at the points in the electrode where the gas to be oxidized or reduced contacts both the electrolyte and the active material of the electrode. In the case of gas generation, electrolyte contacts the active material and gas is generated at this interface.
Electrochemical cells utilizing such electrodes generally comprise the gas diffusion electrode, a spaced counter electrode, a liquid electrolyte (which is generally aqueous) which contacts both the counter electrode and the gas diffusion electrode, and a gas which contacts the gas diffusion electrode either (1) for reduction or oxidation of the gas or (2) produced via electrolytic generation. Circuit connections are disposed between the counter and gas diffusion electrodes. Additionally, the counter electrode may also be a gas diffusion electrode. A well known example of such a design is the H.sub.2 /O.sub.2 fuel cell.
Electrochemical batteries, for example, the metal-air type, commonly utilize either an aqueous alkaline or neutral (e.g., saline) electrolyte, while fuel cells may commonly utilize either acidic electrolytes or alkaline electrolytes. Other types of electrolytes are also used, depending upon the specific gas which is consumed or generated.
The use in electrochemical batteries of an oxygen containing gas such as air which is reduced at the gas diffusion electrode is well known. However, the gas need not be oxygen-containing nor need it be reduced at the gas diffusion electrode. For example, hydrogen gas is oxidized in some fuel cells. The present invention is generally applicable to all such types of gas diffusion electrodes and cells.
The electronically conductive material in a gas diffusion electrode typically may be active carbon or carbon black. The carbon also serves as a support for catalysts such as platinum or transition metal organometallic catalysts (such as porphyrins).
In various applications, it is desirable that the liquid electrolyte flow through the body of the cell over the electrode surfaces. The gaseous reactant also typically flows across the outside surface of the electrode in a separate chamber or manifold.
In operation, it is desirable to maintain the potential of the electrode at a level as close as possible to the open circuit potential (OCP), while maintaining as high a current density as possible. For example, it may be desired to operate a cell at a current density of up to as high as 1.0 A/cm.sup.2, but usually somewhat less, while minimizing deviation from the OCP (also termed polarization). For an air cathode, a polarization of up to about 0.20 V at 1.0 A/cm.sup.2 would be considered quite good.
Such voltage losses are exclusive of further voltage losses due to the ohmic drop of the electrolyte and any separator which might be used.
For any given electrode, it would be desirable to improve the polarization behavior at useful current densities. For example, in the case of a so-called air cathode, in which air or another oxygen-containing gas is reduced at a gas diffusion cathode, it is desirable to improve the oxygen reduction polarization behavior of such electrodes, especially at low current densities up to about 100 mA/cm.sup.2. Improvement is noted by a shift of potential of the electrode to more positive levels at any given current density.